WO2010010512A1 - Phosphor-enhanced light source and a luminescent material for use in a phosphor-enhanced light source - Google Patents

Phosphor-enhanced light source and a luminescent material for use in a phosphor-enhanced light source Download PDF

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Publication number
WO2010010512A1
WO2010010512A1 PCT/IB2009/053158 IB2009053158W WO2010010512A1 WO 2010010512 A1 WO2010010512 A1 WO 2010010512A1 IB 2009053158 W IB2009053158 W IB 2009053158W WO 2010010512 A1 WO2010010512 A1 WO 2010010512A1
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Prior art keywords
light
excitation
phosphor
wavelength
luminescent material
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PCT/IB2009/053158
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French (fr)
Inventor
Wouter J. M. Schrama
Chrysostomus H. M. Maree
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Koninklijke Philips Electronics N.V.
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Publication of WO2010010512A1 publication Critical patent/WO2010010512A1/en

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    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K11/00—Luminescent, e.g. electroluminescent, chemiluminescent materials
    • C09K11/08—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials
    • C09K11/77—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials containing rare earth metals
    • C09K11/7728—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials containing rare earth metals containing europium
    • C09K11/7736—Vanadates; Chromates; Molybdates; Tungstates
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K11/00—Luminescent, e.g. electroluminescent, chemiluminescent materials
    • C09K11/08—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials
    • C09K11/77—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials containing rare earth metals
    • C09K11/7783—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials containing rare earth metals containing two or more rare earth metals one of which being europium
    • C09K11/7794—Vanadates; Chromates; Molybdates; Tungstates
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/48—Semiconductor devices with at least one potential-jump barrier or surface barrier specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor body packages
    • H01L33/50—Wavelength conversion elements
    • H01L33/501—Wavelength conversion elements characterised by the materials, e.g. binder
    • H01L33/502—Wavelength conversion materials

Definitions

  • the invention relates to a phosphor-enhanced light source.
  • the invention further relates to a luminescent material for use in a phosphor- enhanced light source.
  • Phosphor-enhanced light sources are known per se and are used for substantially all kinds of light sources.
  • Phosphor-enhanced light sources comprise a light emitter and a luminescent material.
  • the luminescent material is arranged for converting at least part of the light emitted by the light emitter into light of a longer wavelength.
  • Well-known phosphor-enhanced light sources are, for example, mercury vapor discharge lamps in which the light emitter is a discharge vessel in which the presence of mercury vapor causes the discharge to emit ultraviolet radiation. At least a part of the ultraviolet radiation is absorbed by luminescent material and converted into light of a longer wavelength which is subsequently emitted by the luminescent material.
  • a mercury vapor discharge lamp may, for example, comprise a discharge vessel in which the discharge is generated.
  • the luminescent material is typically applied on the inner wall of the discharge vessel such that the ultraviolet radiation emitted by the discharge does not need to pass the discharge vessel but is converted inside the discharge vessel into, for example, visible light.
  • the phosphor-enhanced light source may, for example, comprise a solid-state light emitter as the light emitter.
  • a solid-state light emitter may, for example, be a light emitting diode, or a laser diode, or an organic light emitting diode.
  • the light emitted by a solid-state light emitter typically has a relatively narrow spectrum arranged around a center wavelength.
  • the width of the spectrum may, for example, be defined by the Full Width Half Maximum (further also indicated as FWHM) of the emission peak, which is a width of the emission peak measured at an intensity being half the maximum emission intensity of the light emitted by the solid-state light emitter.
  • the FWHM of a typical emission spectrum of the solid-state light emitter is less than 30 nanometer, which is typically identified by the human eye as light of a single color.
  • luminescent materials may be added to generate a phosphor- enhanced light source.
  • the luminescent material may, for example, be applied as a layer on top of the die of the solid-state light emitter, or may, for example, be dispersed in a matrix.
  • the luminescent material may also be part of a mixture of different luminescent materials, for example, each generating a different color such that the mixed light, for example, generates white light having a specific color temperature.
  • luminescent materials may be added to solid-state light emitters to improve the color rendering characteristics of the solid-state light emitters, as the typical emission characteristic of the luminescent materials includes a relatively broad spectrum of light.
  • the luminescent material preferably is applied directly on the die of the solid-state light emitter, because in such an arrangement the phosphor-enhanced light source substantially maintains its point-source characteristic, which is beneficial when designing the optical system around the phosphor- enhanced light source.
  • a drawback of applying the luminescent material directly on the die of the solid-state light emitter is that the luminescent material becomes relatively hot, which reduces the efficiency of most luminescent materials.
  • This reduction of the efficiency is caused by thermal quenching, which reversibly reduces the efficiency and is caused by (partial) decomposition of the luminescent material, which irreversibly reduces the efficiency of the phosphor-enhanced light source.
  • Some of the known luminescent materials which can withstand the harsh environment on top of the die of the solid-state light emitter typically require ultraviolet radiation as excitation radiation.
  • Solid-state light emitters emitting ultraviolet radiation are however relatively difficult to produce and have a less efficient light generation compared to solid-state light emitters which emit near-ultraviolet, visible violet or visible blue radiation.
  • a phosphor-enhanced light source having luminescent material which can withstand relatively high temperatures, which phosphor-enhanced light source exhibits improved efficiency.
  • the object is achieved with a phosphor-enhanced light source comprising: a light emitter emitting excitation light having a predefined spectral range around a predefined wavelength, and a luminescent material for absorbing at least a part of the excitation light and for converting at least a part of the absorbed excitation light into emission light of a longer wavelength compared to the excitation light, the luminescent material comprising:
  • the luminescent material (Yi_ x _ y _ z Gd x Bi y Eu z )(Vi_ a _ b B a P b )O 4 , with 0 ⁇ x ⁇ 0.999, 0 ⁇ y ⁇ 0.999, 0.001 ⁇ z ⁇ 0.20 and x + y + z ⁇ 1, and 0 ⁇ a ⁇ 1, 0 ⁇ b ⁇ 1 and a + b ⁇ 1 is indicated, for short, as YVO.
  • the inventors have found that the excitation spectrum of YVO shifts to longer wavelengths at higher temperatures.
  • This unexpected effect is used in the phosphor-enhanced light source according to the invention, in that the light emitter used for exciting the luminescent material emits light of a wavelength substantially outside the excitation spectrum of the luminescent material at room temperature.
  • the wavelength of the light with which the luminescent material is excited is longer compared to the maximum excitation wavelength of the luminescent material at room temperature. This is possible due to the shift of the excitation spectrum of YVO at high temperatures.
  • the excitation spectrum comprises a range of excitation wavelengths which may be used for exciting YVO. This range of excitation wavelengths typically is relatively broad.
  • the location of the excitation spectrum may be defined by an edge of the excitation spectrum corresponding with the maximum excitation wavelength being the excitation wavelength at half the maximum intensity at which YVO may be excited. It has been found that the edge of the excitation spectrum shifts to longer wavelengths when the temperature of YVO is increased.
  • the wavelength of the light emitted by the light emitter for exciting the luminescent material preferably comprises light below the maximum excitation wavelength. This is because the excitation efficiency near the edge of the excitation band changes significantly when the wavelength of the light emitter changes only little, for example, due to temperature changes in the light emitter. Such a significant change due to a relatively small variation in the wavelength of the excitation light would cause the light intensity of the phosphor-enhanced light source to be relatively unpredictable, which, of course, is undesirable.
  • the luminescent material in operation, has an operational temperature at or above 80 degrees Celsius and further comprises YVO.
  • the wavelength of the light with which the luminescent material is irradiated to excite YVO is longer compared to the maximum excitation wavelength of YVO at room temperature, which typically is approximately 25 degrees Celsius.
  • the wavelength of the light with which YVO at or above 80 degrees Celsius is excited is outside the excitation band of YVO at room temperature.
  • a further benefit of the use of YVO in a phosphor-enhanced light source, in which the luminescent materialhas a temperature, in operation, at or above 80 degrees Celsius, is that YVO has a very good thermal stability and the efficiency of YVO increases when the temperature increases at certain excitation wavelengths. Furthermore, the emission spectrum of YVO is relatively narrow and substantially wholly within the range of visible light, further enhancing the efficiency when using YVO for generating visible light. Due to the shift in excitation wavelength at high temperatures for YVO according to the current invention, the efficiency of the phosphor-enhanced light source comprising YVO may be even further enhanced.
  • a still further benefit of the phosphor-enhanced light source according to the invention is that the predefined wavelength of the excitation light may even be within the range of visible light. As a result, part of the excitation light may contribute to the visible light emitted by the phosphor-enhanced light source without having to be converted via the luminescent material or a further luminescent material into light of a longer wavelength. Omitting the need for converting part of the excitation wavelength further enhances the efficiency of the phosphor-enhanced light source.
  • the predefined wavelength may, for example, be visible light of the color blue which may be used together with luminescent materials converting the remainder of the excitation light into red light and green light or into yellow light to obtain a phosphor-enhanced light source emitting substantially white light.
  • the light emitter may be any light source emitting light having the predefined spectrum, for example, a low pressure discharge lamp, a high pressure discharge lamp, an incandescent lamp, a solid-state light emitter, or even a further luminescent material emitting the excitation light having the predefined spectral range around the predefined wavelength.
  • the predefined wavelength of the excitation light is at least 10 nanometer longer compared to the maximum excitation wavelength of YVO at room temperature.
  • the light emitter is a solid-state light emitter.
  • Solid-state light emitters comprise light emitting diodes (further also referred to as LED), laser diodes (further also referred to as LD), and organic light emitting diodes (further also referred to as OLED).
  • YVO is especially beneficial because solid-state light emitters which emit light having a longer wavelength are generally more efficient compared to solid-state light emitters emitting light of a shorter wavelength. Especially when the excitation light is in the range of ultraviolet light, the efficiency of the solid-state light emitters is relatively poor.
  • excitation light having an increased wavelength compared to the maximum excitation wavelength of YVO at room temperature immediately increases the efficiency of the phosphor-enhanced light source due to the improvement of the efficiency of the solid-state light emitter. Furthermore, when the luminescent material is applied directly on the die of the solid-state light emitter the luminescent material typically becomes relatively hot, which is a drawback for most luminescent materials.
  • the increased temperature of YVO is beneficial both for its light emission efficiency and for its shift of the excitation spectrum enabling excitation light of a longer wavelength compared to the maximum excitation wavelength of YVO at room temperature.
  • the solid-state light emitter comprises a die for emitting light, the die having, in operation, an operating temperature at or above 80 degrees Celsius.
  • a solid-state light emitter in which the die, in operation, has an operating temperature at or above 80 degrees Celsius is also known as a high-power solid-state light emitter.
  • the temperature of the die of such a high-power solid- state light emitter may become very hot - up to, for example, 200 to 300 degrees Celsius. This seriously limits the range of luminescent materials which can withstand these harsh environments on or near such high-power solid-state light emitters.
  • YVO on such high-power solid-state light emitter causes, in operation, the operational temperature of YVO to be way above 80 degrees Celsius, which further improves the efficiency of YVO and which further shifts the excitation spectrum to longer wavelengths compared to the maximum excitation wavelength of YVO at room temperature. This makes the application of YVO at solid-state light emitters operating at high temperatures very beneficial.
  • YVO may be directly applied on the die of the high-power solid-state light emitter.
  • the substantially point-source behavior of the high-power solid-state light emitter is maintained in the phosphor-enhanced light source, enabling a relatively simple optical system for guiding and shaping the light emitted from the substantially point-source like phosphor-enhanced light source.
  • the manufacturing of the phosphor-enhanced light source comprising a high-power solid-state light emitter as light emitter may be relatively simple, as the luminescent material comprising YVO may simply be applied directly on the die of the high-power solid-state light emitter.
  • the operational temperature of the luminescent material is at or above 150 degrees Celsius. Only very few luminescent materials have sufficient thermal stability to be used at temperatures at or above 150 degrees Celsius.
  • YVO is not only thermally stable at such temperatures, but the efficiency of YVO at such high temperatures is further improved at certain excitation wavelengths. Combining this with a further shift of the excitation spectrum, enabling the predefined wavelength to be even longer, the efficiency of the phosphor- enhanced light source is even further improved.
  • the operational temperature of the luminescent material in operation, is at or above 200 degrees Celsius.
  • the inventors have found that the optimum efficiency of, for example, luminescent material comprising YVO 4 IEu 3+ lies between 200 and 350 degrees Celsius when the predefined wavelength is 350 nanometer.
  • YVO 4 IEu 3+ is one of the luminescent materials which fall within the chemical formula of YVO.
  • the maximum excitation wavelength of YVO 4 :Eu 3+ at 25 degrees Celsius is 338 nanometer.
  • YVO constitutes a layer on top of the light emitter, or is dispersed in a matrix material, or is mixed with other luminescent materials, or constitutes a lumiramic, or is arranged as a remote phosphor.
  • a lumiramic comprises a ceramic layer which is composed of, or which includes, a wavelength-converting material such as a luminescent material.
  • the lumiramic also known as luminescent ceramic layer, typically is more robust and less sensitive to relatively high temperatures compared to other methods of applying luminescent material. Furthermore, luminescent ceramics exhibit less scattering and may therefore increase the conversion efficiency more than other methods of applying luminescent material.
  • the luminescent material comprises YVO selected from a group comprising:
  • the luminescent material (Y0.9sEu0.02) VO 4 , also indicated as YVO 4 :Eu + , is used as reference in the remainder of this document.
  • the addition of other materials changes the excitation and/or emission spectrum of YVO 4 :Eu 3+ , which may be used to tune the luminescent material to suit the exact purpose in a specific application.
  • the addition of Bismuth (Bi) to YVO 4 :Eu 3+ alters the excitation spectrum of YVO 4 :Eu 3+ and shifts the excitation spectrum towards longer wavelengths compared to YVO 4 :Eu 3+ .
  • adding, for example, Bismuth may result in a luminescent material indicated as (Yo.93Bio.o5Euo.o2)(Vo.8Po.2)0 4 and may cause a shift of the maximum excitation wavelength due to the added Bismuth, and a further shift of the maximum excitation wavelength due to the relative operating temperature that is at or above 80 degrees Celsius.
  • the invention also relates to the use of (Yi_ x _y_ z Gd x Bi y Eu z )(Vi_ a _bB a Pb)O 4 , with 0 ⁇ x ⁇ 0.999, 0 ⁇ y ⁇ 0.999, 0.001 ⁇ z ⁇ 0.20 and x + y + z ⁇ 1, and 0 ⁇ a ⁇ 1, 0 ⁇ b ⁇ 1 and a + b ⁇ 1, in a phosphor-enhanced light source comprising a light emitter emitting excitation light having a predefined spectral range around a predefined wavelength, in operation, YVO having an operational temperature at or above 80 degrees Celsius, and the predefined wavelength of the excitation light being longer compared to a maximum excitation wavelength of YVO at room temperature.
  • a phosphor-enhanced light source comprising a light emitter emitting excitation light having a predefined spectral range around a predefined wavelength
  • Figs. IA and IB show excitation spectra and emission spectra of YVO 4 :Eu 3+ being a luminescent material falling within the chemical composition
  • Fig. 3 shows emission intensities versus temperature of YVO 4 :Eu + for different predefined wavelengths of the excitation light
  • Figs. 4A and 4B show an embodiment of a phosphor-enhanced light source according to the invention.
  • Figs. IA and IB show excitation spectra and emission spectra of YVO 4 :Eu + being a luminescent material 30 (see Figs. 4A and 4B) falling within the chemical composition (Yi_ x _ y _ z Gd x Bi y Eu z )(Vi_a-bB a P b )O 4 , with 0 ⁇ x ⁇ 0.999, 0 ⁇ y ⁇ 0.999, 0.001 ⁇ z ⁇ 0.20 and x + y + z ⁇ 1 , and 0 ⁇ a ⁇ 1 , 0 ⁇ b ⁇ 1 and a + b ⁇ 1.
  • the luminescent material (Yi_ x _ y _ z Gd x Bi y Eu z )(Vi_a-bB a P b )O 4 with 0 ⁇ x ⁇ 0.999, 0 ⁇ y ⁇ 0.999,
  • IA shows excitation spectra OfYVO 4 IEu 3+ .
  • the excitation spectra shown in Fig. IA represent the emission OfYVO 4 IEu + measured at a measurement wavelength of 619 nanometer, while the predefined wavelength ⁇ p of the excitation light hvl (see Fig. 4B) is changed from 200 nanometer to 500 nanometer.
  • the excitation spectra shown in Fig. IA are measured at different temperatures of the luminescent material 30 YVO 4 :Eu 3+ ranging from 25 degrees Celsius, which is defined as room temperature, to 330 degrees Celsius. From these excitation spectra the efficiency of the absorption of the excitation light hvl can be derived at different wavelengths. Furthermore, from the excitation spectra shown in Fig.
  • the excitation spectra at high temperatures are shifted toward longer wavelengths.
  • the predefined wavelength ⁇ p is 350 nanometer, which is approximately 12 nanometer above the maximum excitation wavelength ⁇ max (being 338 nanometer) of YVO 4 :Eu 3+ at room temperature (as can be seen from Fig. IA).
  • An upper limit of the excitation band ⁇ exc of the excitation spectrum of a luminescent material YVO 4 :Eu + is typically defined by an edge wavelength ⁇ max which corresponds to the maximum excitation wavelength ⁇ max at which the excitation intensity is half of the maximum excitation intensity.
  • the predefined wavelength ⁇ p of the excitation light hvl for exciting the luminescent material 30 YVO 4 :Eu + is preferably chosen below the maximum ⁇ max excitation wavelength of the excitation spectrum, and even more preferably at some distance from the maximum ⁇ max excitation wavelength.
  • the luminescent material 20 in operation, has an operational temperature at or above 80 degrees Celsius and further comprises YVO 4 :Eu 3+ .
  • the predefined wavelength ⁇ p with which the luminescent material 30 is irradiated to excite the YVO 4 :Eu 3+ is longer compared to the maximum excitation wavelength ⁇ max of YVO 4 :Eu 3+ at room temperature, which typically is approximately 25 degrees Celsius.
  • the phosphor-enhanced light source 10 may use light having a longer wavelength for exciting the YVO 4 :Eu 3+ and as a result reduces the Stokes shift in the phosphor-enhanced light source 10, thus improving the efficiency.
  • Fig. IB further shows the emission spectra of YVO 4 :Eu 3+ at different temperatures of YVO 4 :Eu 3+ .
  • the emission spectra of YVO 4 :Eu 3+ show the relatively narrow emission peak at about 620 nanometer which is substantially fully within the range of visible light.
  • the emission spectra of YVO 4 :Eu 3+ as shown in Fig. IB have been measured using a predefined wavelength ⁇ p of the excitation light hvl being 254 nanometer. As can clearly be seen in Fig.
  • the spectra of the emission light hv2 at different temperatures of YVO 4 :Eu 3+ substantially fully overlap and thus there is no shift in the emission spectrum of YVO 4 :Eu 3+ similar to the shift seen in the excitation spectrum of YVO 4 :Eu 3+ when the temperature is increased.
  • the absorption efficiency and subsequently the emission intensity of the YVO 4 :Eu 3+ luminescent material 30 increases when the temperature is increased.
  • the excitation spectra of YVO 4 :Eu 3+ in Fig. 2 are normalized to further clarify the shift of the excitation spectra with temperature.
  • the predefined wavelength ⁇ p is indicated and the efficiency increase with temperature is shown by adding additional arrows to the arrow indicated at the predefined wavelength ⁇ p being 350 nanometer.
  • the predefined wavelength ⁇ p of the excitation light hvl lies substantially above the maximum ⁇ max excitation wavelength of YVO 4 :Eu + at room temperature. This is especially beneficial when using a solid-state light emitter 20 as light emitter 20 in the phosphor-enhanced light source 10, because the efficiency of the light generated in the solid-state light emitter 20 typically increases when the wavelength is increased.
  • the predefined wavelength ⁇ p of the excitation light hvl may be increased while maintaining a relatively efficient excitation of YVO, which increases the efficiency of the solid-state light emitter 20 and the efficiency of the luminescent material 30.
  • Fig. 3 shows emission intensities versus temperature OfYVO 4 IEu 3+ for different predefined wavelengths ⁇ p of the excitation light hvl (see Fig. 4B). From Fig. 3 it is clear that when exciting YVO 4 IEu 3+ with excitation light hvl having a predefined wavelength ⁇ p well within the excitation range ⁇ exc of YVO 4 :Eu 3+ at room temperature (diamond-shaped dots and square dots in Fig. 3), the efficiency of YVO 4 :Eu 3+ is reduced when increasing the temperature of YVO 4 :Eu 3+ , said expected reduction in efficiency being due to thermal quenching effects.
  • YVO 4 :Eu 3+ luminescent material has a temperature of approximately 330 degrees Celsius. Other compositions of YVO may achieve maximum efficiency at a different temperature.
  • Figs. 4A and 4B show an embodiment of a phosphor-enhanced light source 10 according to the invention, with Fig. 4B showing the individual elements 20, 30, 40 of the phosphor-enhanced light source 10.
  • the phosphor-enhanced light source 10 comprises a light emitter 20 and a luminescent material 30.
  • the phosphor-enhanced light source 10 as shown in Figs. 4A and 4B comprises a lens 40 for shaping the emission light hv2 to a predefined emission profile of the phosphor-enhanced light source 10.
  • the luminescent material 30 is arranged on top of the light emitter 20. Because the light emitter 20 typically not only generates excitation light hvl, but also heat, which increases the temperature of the luminescent material 30. Generally the efficiency decreases due to an increase of the temperature of the luminescent materials 30. However, the YVO luminescent material has an improved heat resistance and improved efficiency at high temperatures. Furthermore, as is disclosed in the current application, the excitation wavelength of the YVO luminescent material increases when the temperature increases, enabling the excitation light hvl to have a longer predefined wavelength ⁇ p than expected. Due to this unexpected increase of the predefined wavelength ⁇ p at elevated temperatures, the efficiency of the phosphor-enhanced light sources 10 at high temperatures is increased.
  • the increase in efficiency when YVO is used as a luminescent material is significant, apart from the benefit that the YVO luminescent material may simply be applied on top of the light emitter 20, for example, on a light emitting die 22 of a solid-state light emitter 20.
  • This provides, in addition to the mentioned energy efficiency benefits, also manufacturing benefits in that this arrangement of the luminescent material 30 is relatively simple to achieve in a production process.
  • the light emitter 20 is a solid- state light emitter 20, for example, a light emitting diode, a laser diode or an organic light emitting diode.
  • a solid- state light emitter 20 for example, a light emitting diode, a laser diode or an organic light emitting diode.
  • the heat production of the solid-state light emitter 20 causes the luminescent material 30 to be heated sufficiently to obtain the efficiency improvement according to the invention.
  • the light emitter 20 may be any light emitter 20 which can be used for emitting the excitation light hvl as defined above.
  • Such an alternative light emitter 20 may, for example, be a high pressure or low pressure mercury vapor discharge lamp, an incandescent lamp, a further luminescent material emitting the excitation light hvl or any other light emitter 20 which emits the excitation light hvl .
  • any reference signs placed between parentheses shall not be construed as limiting the claim.
  • Use of the verb "comprise” and its conjugations does not exclude the presence of elements or steps other than those stated in a claim.
  • the article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.
  • the invention may be implemented by means of hardware comprising several distinct elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Abstract

The invention relates to a phosphor-enhanced light source (10) comprising a light emitter (20) and a luminescent material (30). The light emitter emits excitation light (hv1) having a predefined spectral range around a predefined wavelength (λp). The luminescent material absorbs at least a part of the excitation light and converts at least a part of the absorbed excitation light into emission light of a longer wavelength compared to the excitation light. The luminescent material comprises: (Y1-x-y-zGdxBiyEuz)(V1-a-bBaPb)O4 with 0 ≤ x < 0.999, 0 ≤ y < 0.999, 0.001 < z < 0.20 and x + y + z < 1, and 0 ≤ a ≤1, 0 ≤ b ≤1 and a + b < 1, and, in operation, an operational temperature of luminescent material is at or above 80 degrees Celsius. The predefined wavelength of the excitation light is longer compared to a maximum excitation wavelength of (Y1-x-y-zGdxBiyEuz)(V1-a-bBaPb)O4 at room temperature. The invention also relates to the use of (Y1-x-y-zGdxBiyEuz)(V1-a-bBaPb)O4, with 0 ≤ x < 0.999, 0 ≤ y < 0.999, 0.001 < z < 0.20 and x + y + z < 1, and 0 ≤ a ≤1, 0 ≤ b ≤1 and a + b < 1, in a phosphor-enhanced light source.

Description

Phosphor-enhanced light source and a luminescent material for use in a phosphor-enhanced light source
FIELD OF THE INVENTION:
The invention relates to a phosphor-enhanced light source. The invention further relates to a luminescent material for use in a phosphor- enhanced light source.
BACKGROUND OF THE INVENTION:
Phosphor-enhanced light sources are known per se and are used for substantially all kinds of light sources. Phosphor-enhanced light sources comprise a light emitter and a luminescent material. The luminescent material is arranged for converting at least part of the light emitted by the light emitter into light of a longer wavelength.
Well-known phosphor-enhanced light sources are, for example, mercury vapor discharge lamps in which the light emitter is a discharge vessel in which the presence of mercury vapor causes the discharge to emit ultraviolet radiation. At least a part of the ultraviolet radiation is absorbed by luminescent material and converted into light of a longer wavelength which is subsequently emitted by the luminescent material. Such a mercury vapor discharge lamp may, for example, comprise a discharge vessel in which the discharge is generated. The luminescent material is typically applied on the inner wall of the discharge vessel such that the ultraviolet radiation emitted by the discharge does not need to pass the discharge vessel but is converted inside the discharge vessel into, for example, visible light. Alternatively, the phosphor-enhanced light source may, for example, comprise a solid-state light emitter as the light emitter. Such a solid-state light emitter may, for example, be a light emitting diode, or a laser diode, or an organic light emitting diode. The light emitted by a solid-state light emitter typically has a relatively narrow spectrum arranged around a center wavelength. The width of the spectrum may, for example, be defined by the Full Width Half Maximum (further also indicated as FWHM) of the emission peak, which is a width of the emission peak measured at an intensity being half the maximum emission intensity of the light emitted by the solid-state light emitter. The FWHM of a typical emission spectrum of the solid-state light emitter is less than 30 nanometer, which is typically identified by the human eye as light of a single color. To change the color of the light emitted by the solid-state light emitter, luminescent materials may be added to generate a phosphor- enhanced light source. The luminescent material may, for example, be applied as a layer on top of the die of the solid-state light emitter, or may, for example, be dispersed in a matrix. The luminescent material may also be part of a mixture of different luminescent materials, for example, each generating a different color such that the mixed light, for example, generates white light having a specific color temperature. Furthermore, luminescent materials may be added to solid-state light emitters to improve the color rendering characteristics of the solid-state light emitters, as the typical emission characteristic of the luminescent materials includes a relatively broad spectrum of light. In many applications of solid-state light emitters, the luminescent material preferably is applied directly on the die of the solid-state light emitter, because in such an arrangement the phosphor-enhanced light source substantially maintains its point-source characteristic, which is beneficial when designing the optical system around the phosphor- enhanced light source. However, a drawback of applying the luminescent material directly on the die of the solid-state light emitter is that the luminescent material becomes relatively hot, which reduces the efficiency of most luminescent materials. This reduction of the efficiency is caused by thermal quenching, which reversibly reduces the efficiency and is caused by (partial) decomposition of the luminescent material, which irreversibly reduces the efficiency of the phosphor-enhanced light source. Some of the known luminescent materials which can withstand the harsh environment on top of the die of the solid-state light emitter typically require ultraviolet radiation as excitation radiation. Solid-state light emitters emitting ultraviolet radiation are however relatively difficult to produce and have a less efficient light generation compared to solid-state light emitters which emit near-ultraviolet, visible violet or visible blue radiation.
SUMMARY OF THE INVENTION:
It is an object of the invention to provide a phosphor-enhanced light source having luminescent material which can withstand relatively high temperatures, which phosphor-enhanced light source exhibits improved efficiency. According to a first aspect of the invention the object is achieved with a phosphor-enhanced light source comprising: a light emitter emitting excitation light having a predefined spectral range around a predefined wavelength, and a luminescent material for absorbing at least a part of the excitation light and for converting at least a part of the absorbed excitation light into emission light of a longer wavelength compared to the excitation light, the luminescent material comprising:
(Yi_x_y_zGdxBiyEuz)(Vi_a_bBaPb)O4 with 0 < x < 0.999, 0 < y < 0.999, 0.001 < z < 0.20 and x + y + z < 1, and 0 < a <1, 0 < b <1 and a + b < 1, and, in operation, an operational temperature of the luminescent material being at or above 80 degrees Celsius, the predefined wavelength of the excitation light being longer compared to a maximum excitation wavelength of the (Yi_x_y_zGdxBiyEuz)(Vi_a_bBaPb)θ4 at room temperature.
In the remainder of the text, the luminescent material (Yi_x_y_zGdxBiyEuz)(Vi_a_ bBaPb)O4, with 0 < x < 0.999, 0 < y < 0.999, 0.001 < z < 0.20 and x + y + z < 1, and 0 < a <1, 0 < b <1 and a + b < 1 is indicated, for short, as YVO.
While not wishing to be held to any particular theory, the inventors have found that the excitation spectrum of YVO shifts to longer wavelengths at higher temperatures. This unexpected effect is used in the phosphor-enhanced light source according to the invention, in that the light emitter used for exciting the luminescent material emits light of a wavelength substantially outside the excitation spectrum of the luminescent material at room temperature. The wavelength of the light with which the luminescent material is excited is longer compared to the maximum excitation wavelength of the luminescent material at room temperature. This is possible due to the shift of the excitation spectrum of YVO at high temperatures. As it is possible to increase the wavelength of the excitation light to longer values compared to the maximum excitation wavelength of the luminescent material at room temperature, the Stokes shift is reduced, which improves the efficiency of the phosphor- enhanced light source. The excitation spectrum comprises a range of excitation wavelengths which may be used for exciting YVO. This range of excitation wavelengths typically is relatively broad. The location of the excitation spectrum may be defined by an edge of the excitation spectrum corresponding with the maximum excitation wavelength being the excitation wavelength at half the maximum intensity at which YVO may be excited. It has been found that the edge of the excitation spectrum shifts to longer wavelengths when the temperature of YVO is increased. When using a luminescent material having a specific excitation spectrum, the wavelength of the light emitted by the light emitter for exciting the luminescent material preferably comprises light below the maximum excitation wavelength. This is because the excitation efficiency near the edge of the excitation band changes significantly when the wavelength of the light emitter changes only little, for example, due to temperature changes in the light emitter. Such a significant change due to a relatively small variation in the wavelength of the excitation light would cause the light intensity of the phosphor-enhanced light source to be relatively unpredictable, which, of course, is undesirable. In the phosphor-enhanced light source according to the invention the luminescent material, in operation, has an operational temperature at or above 80 degrees Celsius and further comprises YVO. The wavelength of the light with which the luminescent material is irradiated to excite YVO is longer compared to the maximum excitation wavelength of YVO at room temperature, which typically is approximately 25 degrees Celsius. In other words, the wavelength of the light with which YVO at or above 80 degrees Celsius is excited is outside the excitation band of YVO at room temperature. Thus, by exploiting the effect that the excitation spectrum of YVO shifts to longer wavelengths at higher temperatures, the phosphor-enhanced light source according to the invention may use light having a longer wavelength for exciting YVO and thereby reduces the Stokes shift in the phosphor-enhanced light source, thus improving the efficiency.
A further benefit of the use of YVO in a phosphor-enhanced light source, in which the luminescent materialhas a temperature, in operation, at or above 80 degrees Celsius, is that YVO has a very good thermal stability and the efficiency of YVO increases when the temperature increases at certain excitation wavelengths. Furthermore, the emission spectrum of YVO is relatively narrow and substantially wholly within the range of visible light, further enhancing the efficiency when using YVO for generating visible light. Due to the shift in excitation wavelength at high temperatures for YVO according to the current invention, the efficiency of the phosphor-enhanced light source comprising YVO may be even further enhanced. A still further benefit of the phosphor-enhanced light source according to the invention is that the predefined wavelength of the excitation light may even be within the range of visible light. As a result, part of the excitation light may contribute to the visible light emitted by the phosphor-enhanced light source without having to be converted via the luminescent material or a further luminescent material into light of a longer wavelength. Omitting the need for converting part of the excitation wavelength further enhances the efficiency of the phosphor-enhanced light source. The predefined wavelength may, for example, be visible light of the color blue which may be used together with luminescent materials converting the remainder of the excitation light into red light and green light or into yellow light to obtain a phosphor-enhanced light source emitting substantially white light. The light emitter may be any light source emitting light having the predefined spectrum, for example, a low pressure discharge lamp, a high pressure discharge lamp, an incandescent lamp, a solid-state light emitter, or even a further luminescent material emitting the excitation light having the predefined spectral range around the predefined wavelength. In an embodiment of the phosphor-enhanced light source, the predefined wavelength of the excitation light is at least 10 nanometer longer compared to the maximum excitation wavelength of YVO at room temperature.
In an embodiment of the phosphor-enhanced light source, the light emitter is a solid-state light emitter. Solid-state light emitters comprise light emitting diodes (further also referred to as LED), laser diodes (further also referred to as LD), and organic light emitting diodes (further also referred to as OLED). The use of YVO is especially beneficial because solid-state light emitters which emit light having a longer wavelength are generally more efficient compared to solid-state light emitters emitting light of a shorter wavelength. Especially when the excitation light is in the range of ultraviolet light, the efficiency of the solid-state light emitters is relatively poor. Using excitation light having an increased wavelength compared to the maximum excitation wavelength of YVO at room temperature, immediately increases the efficiency of the phosphor-enhanced light source due to the improvement of the efficiency of the solid-state light emitter. Furthermore, when the luminescent material is applied directly on the die of the solid-state light emitter the luminescent material typically becomes relatively hot, which is a drawback for most luminescent materials. However, for YVO, the increased temperature of YVO is beneficial both for its light emission efficiency and for its shift of the excitation spectrum enabling excitation light of a longer wavelength compared to the maximum excitation wavelength of YVO at room temperature. In an embodiment of the phosphor-enhanced light source, the solid-state light emitter comprises a die for emitting light, the die having, in operation, an operating temperature at or above 80 degrees Celsius. A solid-state light emitter in which the die, in operation, has an operating temperature at or above 80 degrees Celsius is also known as a high-power solid-state light emitter. The temperature of the die of such a high-power solid- state light emitter may become very hot - up to, for example, 200 to 300 degrees Celsius. This seriously limits the range of luminescent materials which can withstand these harsh environments on or near such high-power solid-state light emitters. Using YVO on such high-power solid-state light emitter causes, in operation, the operational temperature of YVO to be way above 80 degrees Celsius, which further improves the efficiency of YVO and which further shifts the excitation spectrum to longer wavelengths compared to the maximum excitation wavelength of YVO at room temperature. This makes the application of YVO at solid-state light emitters operating at high temperatures very beneficial.
Furthermore, a relatively small increase of the excitation wavelength may result in a substantial efficiency increase of the high-power solid-state light emitter.
Especially due to the high power consumption of the high-power solid-state light emitters, a relatively small shift of the predefined wavelength of the high-power solid-state light emitter already provides a substantial increase of the efficiency.
Finally, due to the high thermal stability of YVO, YVO may be directly applied on the die of the high-power solid-state light emitter. By applying YVO directly on the die of the high-power solid-state light emitter, the substantially point-source behavior of the high-power solid-state light emitter is maintained in the phosphor-enhanced light source, enabling a relatively simple optical system for guiding and shaping the light emitted from the substantially point-source like phosphor-enhanced light source. Furthermore, the manufacturing of the phosphor-enhanced light source comprising a high-power solid-state light emitter as light emitter may be relatively simple, as the luminescent material comprising YVO may simply be applied directly on the die of the high-power solid-state light emitter.
In an embodiment of the phosphor-enhanced light source, in operation, the operational temperature of the luminescent material is at or above 150 degrees Celsius. Only very few luminescent materials have sufficient thermal stability to be used at temperatures at or above 150 degrees Celsius. When using YVO, YVO is not only thermally stable at such temperatures, but the efficiency of YVO at such high temperatures is further improved at certain excitation wavelengths. Combining this with a further shift of the excitation spectrum, enabling the predefined wavelength to be even longer, the efficiency of the phosphor- enhanced light source is even further improved.
In an embodiment of the phosphor-enhanced light source, in operation, the operational temperature of the luminescent material is at or above 200 degrees Celsius. The inventors have found that the optimum efficiency of, for example, luminescent material comprising YVO4IEu3+ lies between 200 and 350 degrees Celsius when the predefined wavelength is 350 nanometer. YVO4IEu3+ is one of the luminescent materials which fall within the chemical formula of YVO. The maximum excitation wavelength of YVO4:Eu3+ at 25 degrees Celsius is 338 nanometer.
In an embodiment of the phosphor-enhanced light source, YVO: constitutes a layer on top of the light emitter, or is dispersed in a matrix material, or is mixed with other luminescent materials, or constitutes a lumiramic, or is arranged as a remote phosphor. A lumiramic comprises a ceramic layer which is composed of, or which includes, a wavelength-converting material such as a luminescent material. The lumiramic, also known as luminescent ceramic layer, typically is more robust and less sensitive to relatively high temperatures compared to other methods of applying luminescent material. Furthermore, luminescent ceramics exhibit less scattering and may therefore increase the conversion efficiency more than other methods of applying luminescent material.
In an embodiment of the phosphor-enhanced light source, the luminescent material comprises YVO selected from a group comprising:
Figure imgf000008_0001
(Y0.90Gd0.0sEu0.02) VO4 (Yθ.98EUo.02)(Vo.8Pθ.2)04
(Yo.93Bio.o5Euo.o2)(Vo.8Po.2)04, and
(Yθ.98EUo.02)(Vo.95Bo.05)04.
The luminescent material (Y0.9sEu0.02) VO4, also indicated as YVO4 :Eu +, is used as reference in the remainder of this document. The addition of other materials changes the excitation and/or emission spectrum of YVO4:Eu3+, which may be used to tune the luminescent material to suit the exact purpose in a specific application. For example, the addition of Bismuth (Bi) to YVO4:Eu3+ alters the excitation spectrum of YVO4:Eu3+ and shifts the excitation spectrum towards longer wavelengths compared to YVO4:Eu3+. Thus, adding, for example, Bismuth may result in a luminescent material indicated as (Yo.93Bio.o5Euo.o2)(Vo.8Po.2)04 and may cause a shift of the maximum excitation wavelength due to the added Bismuth, and a further shift of the maximum excitation wavelength due to the relative operating temperature that is at or above 80 degrees Celsius.
The invention also relates to the use of (Yi_x_y_zGdxBiyEuz)(Vi_a_bBaPb)O4, with 0 < x < 0.999, 0 < y < 0.999, 0.001 < z < 0.20 and x + y + z < 1, and 0 < a <1, 0 < b <1 and a + b < 1, in a phosphor-enhanced light source comprising a light emitter emitting excitation light having a predefined spectral range around a predefined wavelength, in operation, YVO having an operational temperature at or above 80 degrees Celsius, and the predefined wavelength of the excitation light being longer compared to a maximum excitation wavelength of YVO at room temperature. BRIEF DESCRIPTION OF THE DRAWINGS:
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. In the drawings:
Figs. IA and IB show excitation spectra and emission spectra of YVO4:Eu3+ being a luminescent material falling within the chemical composition
(Yi_x_y_zGdxBiyEuz)(Vi_a-bBaPb)O4, with 0 < x < 0.999, 0 < y < 0.999, 0.001 < z < 0.20 and x + y + z < l, and O ≤ a ≤l, O < b <l and a + b < 1, Fig. 2 shows the excitation spectra of YVO4:Eu3+ in more detail to more clearly show the shift of the excitation spectra to longer wavelengths with increasing temperature,
Fig. 3 shows emission intensities versus temperature of YVO4 :Eu + for different predefined wavelengths of the excitation light, and Figs. 4A and 4B show an embodiment of a phosphor-enhanced light source according to the invention.
The Figures are purely diagrammatic and not drawn to scale. Particularly for clarity, some dimensions are exaggerated strongly. Similar components in the Figures are denoted by the same reference numerals as much as possible.
DETAILED DESCRIPTION OF EMBODIMENTS:
Figs. IA and IB show excitation spectra and emission spectra of YVO4:Eu + being a luminescent material 30 (see Figs. 4A and 4B) falling within the chemical composition (Yi_x_y_zGdxBiyEuz)(Vi_a-bBaPb)O4, with 0 < x < 0.999, 0 < y < 0.999, 0.001 < z < 0.20 and x + y + z < 1 , and 0 < a <1 , 0 < b <1 and a + b < 1. In the remainder of the text, the luminescent material (Yi_x_y_zGdxBiyEuz)(Vi_a-bBaPb)O4 with 0 < x < 0.999, 0 < y < 0.999,
0.001 < z < 0.20 and x + y + z < 1, and 0 < a <1, 0 < b <1 and a + b < 1 is indicated, for short, as YVO. Both the excitation spectra and emission spectra are measured at different temperatures of the YVO4:Eu + phosphor. As can be seen clearly from Figs. IA and IB, the excitation spectrum of YVO4:Eu3+ shifts towards longer wavelengths at increased temperatures while the emission spectrum of YVO4:Eu3+ remains substantially the same, maintaining an emission peak of the emission light hv2 (see Fig. 4B) near 620 nanometer. Fig. IA shows excitation spectra OfYVO4IEu3+. The excitation spectra shown in Fig. IA represent the emission OfYVO4IEu + measured at a measurement wavelength of 619 nanometer, while the predefined wavelength λp of the excitation light hvl (see Fig. 4B) is changed from 200 nanometer to 500 nanometer. The excitation spectra shown in Fig. IA are measured at different temperatures of the luminescent material 30 YVO4:Eu3+ ranging from 25 degrees Celsius, which is defined as room temperature, to 330 degrees Celsius. From these excitation spectra the efficiency of the absorption of the excitation light hvl can be derived at different wavelengths. Furthermore, from the excitation spectra shown in Fig. IA it can clearly be seen that the excitation spectra at high temperatures are shifted toward longer wavelengths. This makes it possible to efficiently excite the YVO4:Eu + using excitation light hvl having a longer predefined wavelength λp compared to the maximum excitation wavelength λmax of YVO4:Eu + at room temperature. In the current example, the predefined wavelength λp is 350 nanometer, which is approximately 12 nanometer above the maximum excitation wavelength λmax (being 338 nanometer) of YVO4:Eu3+ at room temperature (as can be seen from Fig. IA).
An upper limit of the excitation band Δλexc of the excitation spectrum of a luminescent material YVO4 :Eu + is typically defined by an edge wavelength λmax which corresponds to the maximum excitation wavelength λmax at which the excitation intensity is half of the maximum excitation intensity. The predefined wavelength λp of the excitation light hvl for exciting the luminescent material 30 YVO4:Eu + is preferably chosen below the maximum λmax excitation wavelength of the excitation spectrum, and even more preferably at some distance from the maximum λmax excitation wavelength. This is because the excitation efficiency near the edge λmax of the excitation range changes significantly when the predefined wavelength λp of the light emitter 20 changes only little, for example, due to temperature changes in the light emitter 20. Such a significant change of emission due to a relatively small variation in the predefined wavelength λp of the excitation light hvl would cause the light intensity of a phosphor-enhanced light source 10 (see Figs. 4A and 4B) to be relatively unpredictable, which, of course, is undesirable.
In the phosphor-enhanced light source 10 according to the invention, the luminescent material 20, in operation, has an operational temperature at or above 80 degrees Celsius and further comprises YVO4:Eu3+. The predefined wavelength λp with which the luminescent material 30 is irradiated to excite the YVO4:Eu3+ is longer compared to the maximum excitation wavelength λmax of YVO4:Eu3+ at room temperature, which typically is approximately 25 degrees Celsius. This means that the predefined wavelength λp of the light with which YVO4IEu3+ at or above 80 degrees Celsius is excited is above the maximum λmax excitation wavelength, and thus outside the excitation range ΔλeXC of the excitation spectrum OfYVO4IEu + at room temperature. Thus, by exploiting the unexpected effect that the excitation spectrum of YVO4:Eu3+ shifts to longer wavelengths at higher temperatures, the phosphor-enhanced light source 10 according to the invention may use light having a longer wavelength for exciting the YVO4:Eu3+ and as a result reduces the Stokes shift in the phosphor-enhanced light source 10, thus improving the efficiency.
Fig. IB further shows the emission spectra of YVO4:Eu3+ at different temperatures of YVO4:Eu3+. The emission spectra of YVO4:Eu3+ show the relatively narrow emission peak at about 620 nanometer which is substantially fully within the range of visible light. The emission spectra of YVO4:Eu3+ as shown in Fig. IB have been measured using a predefined wavelength λp of the excitation light hvl being 254 nanometer. As can clearly be seen in Fig. IB, the spectra of the emission light hv2 at different temperatures of YVO4:Eu3+ substantially fully overlap and thus there is no shift in the emission spectrum of YVO4:Eu3+ similar to the shift seen in the excitation spectrum of YVO4:Eu3+ when the temperature is increased. Thus, when choosing the predefined wavelength λp above the maximum λmax excitation wavelength of YVO4 :Eu + at room temperature, the absorption efficiency and subsequently the emission intensity of the YVO4:Eu3+ luminescent material 30 increases when the temperature is increased. Fig. 2 shows the excitation spectra of YVO4:Eu3+ in more detail to more clearly show the shift of the excitation spectra to longer wavelengths with increasing temperature. The excitation spectra of YVO4:Eu3+ in Fig. 2 are normalized to further clarify the shift of the excitation spectra with temperature. Again the predefined wavelength λp is indicated and the efficiency increase with temperature is shown by adding additional arrows to the arrow indicated at the predefined wavelength λp being 350 nanometer. Using this predefined wavelength λp of 350 nanometer, the predefined wavelength λp of the excitation light hvl lies substantially above the maximum λmax excitation wavelength of YVO4:Eu + at room temperature. This is especially beneficial when using a solid-state light emitter 20 as light emitter 20 in the phosphor-enhanced light source 10, because the efficiency of the light generated in the solid-state light emitter 20 typically increases when the wavelength is increased.
So, when using a luminescent material 30 defined by the chemical composition (Yi_x_y_zGdxBiyEuz)(Vi_a-bBaPb)O4, with 0 < x < 0.999, 0 < y < 0.999, 0.001 < z < 0.20 and x + y + z < 1, and 0 < a <1, 0 < b <1 and a + b < 1 at or above 80 degrees Celsius, the predefined wavelength λp of the excitation light hvl may be increased while maintaining a relatively efficient excitation of YVO, which increases the efficiency of the solid-state light emitter 20 and the efficiency of the luminescent material 30.
Fig. 3 shows emission intensities versus temperature OfYVO4IEu3+ for different predefined wavelengths λp of the excitation light hvl (see Fig. 4B). From Fig. 3 it is clear that when exciting YVO4IEu3+ with excitation light hvl having a predefined wavelength λp well within the excitation range Δλexc of YVO4:Eu3+ at room temperature (diamond-shaped dots and square dots in Fig. 3), the efficiency of YVO4:Eu3+ is reduced when increasing the temperature of YVO4:Eu3+, said expected reduction in efficiency being due to thermal quenching effects. However, when exciting YVO4:Eu3+ with excitation light hvl having a predefined wavelength λp longer than the maximum excitation wavelength λmax of YVO4 :Eu + at room temperature (triangular dots in Fig. 3), the efficiency increases when increasing the temperature of YVO4:Eu3+. This unexpected behavior of the YVO luminescent material is used in the current invention to improve the efficiency of the phosphor-enhanced light source 10 according to the invention. Maximum efficiency is obtained when the
YVO4:Eu3+ luminescent material has a temperature of approximately 330 degrees Celsius. Other compositions of YVO may achieve maximum efficiency at a different temperature.
Figs. 4A and 4B show an embodiment of a phosphor-enhanced light source 10 according to the invention, with Fig. 4B showing the individual elements 20, 30, 40 of the phosphor-enhanced light source 10. The phosphor-enhanced light source 10 comprises a light emitter 20 and a luminescent material 30. In addition, the phosphor-enhanced light source 10 as shown in Figs. 4A and 4B comprises a lens 40 for shaping the emission light hv2 to a predefined emission profile of the phosphor-enhanced light source 10.
The luminescent material 30 is arranged on top of the light emitter 20. Because the light emitter 20 typically not only generates excitation light hvl, but also heat, which increases the temperature of the luminescent material 30. Generally the efficiency decreases due to an increase of the temperature of the luminescent materials 30. However, the YVO luminescent material has an improved heat resistance and improved efficiency at high temperatures. Furthermore, as is disclosed in the current application, the excitation wavelength of the YVO luminescent material increases when the temperature increases, enabling the excitation light hvl to have a longer predefined wavelength λp than expected. Due to this unexpected increase of the predefined wavelength λp at elevated temperatures, the efficiency of the phosphor-enhanced light sources 10 at high temperatures is increased. Especially when using high-power light emitters 20, the increase in efficiency when YVO is used as a luminescent material is significant, apart from the benefit that the YVO luminescent material may simply be applied on top of the light emitter 20, for example, on a light emitting die 22 of a solid-state light emitter 20. This provides, in addition to the mentioned energy efficiency benefits, also manufacturing benefits in that this arrangement of the luminescent material 30 is relatively simple to achieve in a production process.
In the embodiment shown in Figs. 4A and 4B, the light emitter 20 is a solid- state light emitter 20, for example, a light emitting diode, a laser diode or an organic light emitting diode. As is explained above, especially when using high-power solid-state light emitters 20, being solid-state light emitters 20 having a die 22 which , in operation, have an operating temperature at or above 80 degrees Celsius, the heat production of the solid-state light emitter 20 causes the luminescent material 30 to be heated sufficiently to obtain the efficiency improvement according to the invention. Furthermore, it will be immediately clear to a person skilled in the art that the light emitter 20 may be any light emitter 20 which can be used for emitting the excitation light hvl as defined above. Such an alternative light emitter 20 may, for example, be a high pressure or low pressure mercury vapor discharge lamp, an incandescent lamp, a further luminescent material emitting the excitation light hvl or any other light emitter 20 which emits the excitation light hvl .
It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims.
In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

CLAIMS:
1. A phosphor-enhanced light source (10) comprising: a light emitter (20) emitting excitation light (hvl) having a predefined spectral range around a predefined wavelength (λp), and a luminescent material (30) for absorbing at least a part of the excitation light (hvl) and for converting at least a part of the absorbed excitation light (hvl) into emission light (hv2) of a longer wavelength compared to the excitation light (hvl), the luminescent material (30) comprising:
(Yi_x_y_zGdxBiyEuz)(Vi_a_bBaPb)O4 with 0 < x < 0.999, 0 < y < 0.999, 0.001 < z < 0.20 and x + y + z < 1, and 0 < a <1, 0 < b <1 and a + b < 1, and, in operation, an operational temperature of the luminescent material (30) being at or above 80 degrees Celsius, the predefined wavelength (λp) of the excitation light (hvl) being longer compared to a maximum excitation wavelength (λmax) of the (Yi_x_y_zGdxBiyEuz)(Vi_a_ bBaPb)θ4 at room temperature.
2. A phosphor-enhanced light source (10) as claimed in claim 1, wherein the predefined wavelength (λp) of the excitation light (hvl) is at least 10 nanometer longer compared to the maximum excitation wavelength (λmax) of the (Yi_x_y_zGdxBiyEuz)(Vi_a_ bBaPb)θ4 at room temperature.
3. A phosphor-enhanced light source (10) as claimed in claim 1 or 2, wherein the light emitter (20) is a solid-state light emitter (20).
4. A phosphor-enhanced light source (10) as claimed in claim 3, wherein the solid-state light emitter (20) comprises a die (22) for emitting light, the die (22) having, in operation, an operating temperature at or above 80 degrees Celsius.
5. Phosphor-enhanced light source (10) as claimed in any of the previous claims, wherein, in operation, the operational temperature of the luminescent material (30) is at or above 150 degrees Celsius.
6. Phosphor-enhanced light source (10) as claimed in any of the previous claims, wherein, in operation, the operational temperature of the luminescent material (30) is at or above 200 degrees Celsius.
7. Phosphor-enhanced light source (10) as claimed in any of the previous claims, wherein (Yi_x_y_zGdxBiyEuz)(Vi_a-bBaPb)O4 with 0 < x < 0.999, 0 < y < 0.999, 0.001 < z < 0.20 and x + y + z < 1, and 0 < a <1, 0 < b <1 and a + b < 1 : constitutes a layer on top of the light emitter, or is dispersed in a matrix material, or is mixed with other luminescent materials, or constitutes a lumiramic, or is arranged as a remote phosphor.
8. Phosphor-enhanced light source (10) as claimed in any of the previous claims, wherein the luminescent material (30) comprises (Yi_x_y_zGdxBiyEuz)(Vi_a_bBaPb)θ4, with 0 < x < 0.999, 0 < y < 0.999, 0.001 < z < 0.20 and x + y + z < 1, and 0 < a <1, 0 < b <1 and a + b < 1 being selected from a group comprising:
Figure imgf000015_0001
(Y0.90Gd0.0sEu0.02) VO4
(Yθ.98EUo.02)(Vo.8Pθ.2)04 (Yo.93Bio.o5Euo.o2)(Vo.8Po.2)θ4, and
(Yθ.98EUo.02)(Vo.95Bo.05)04.
9. Use of (Yi_x_y_zGdxBiyEuz)(Vi_a-bBaPb)O4, with 0 < x < 0.999, 0 < y < 0.999, 0.001 < z < 0.20 and x + y + z < 1, and 0 < a <1, 0 < b <1 and a + b < 1 in a phosphor- enhanced light source (10) comprising a light emitter (20) emitting excitation light (hvl) having a predefined spectral range around a predefined wavelength (λp), in operation, (Y1-x-y- zGdxBiyEuz)(Vi_a_bBaPb)O4 having an operational temperature at or above 80 degrees Celsius, and the predefined wavelength (λp) of the excitation light (hvl) being longer compared to a maximum excitation wavelength (λmax) of (Yi_x_y_zGdxBiyEuz)(Vi_a_bBaPb)θ4 at room temperature.
10. Use of (Yi_x_y_zGdxBiyEuz)(Vi_a-bBaPb)O4, with 0 < x < 0.999, 0 < y < 0.999, 0.001 < z < 0.20 and x + y + z < 1, and 0 < a <1, 0 < b <1 and a + b < 1 as claimed in claim 9, wherein the predefined wavelength (λp) of the excitation light (hvl) is at least 15 nanometer longer compared to the maximum excitation wavelength (λmax) of (Yi_x_y_zGdxBiyEuz)(Vi_a_ bBaPb)θ4 at room temperature.
11. Use of (Yi_x_y_zGdxBiyEuz)(Vi_a_bBaPb)O4, with 0 < x < 0.999, 0 < y < 0.999,
0.001 < z < 0.20 and x + y + z < 1, and 0 < a <1, 0 < b <1 and a + b < 1 as claimed in claim 9 or 10, wherein the light emitter (20) is a solid-state light emitter (20).
PCT/IB2009/053158 2008-07-24 2009-07-21 Phosphor-enhanced light source and a luminescent material for use in a phosphor-enhanced light source WO2010010512A1 (en)

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Publication number Priority date Publication date Assignee Title
EP1106668A2 (en) * 1999-12-01 2001-06-13 General Electric Company Luminescent material doped with trivalent and/or divalent ions
WO2002103748A1 (en) * 2001-06-19 2002-12-27 Koninklijke Philips Electronics N.V. Low-pressure gas discharge lamp with a mercury-free gas filling
WO2004096714A1 (en) * 2003-04-28 2004-11-11 Nanosolutions Gmbh Synthesis of nanoparticles comprising metal (iii) vanadate
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